We trained our Chi to pull on a bell (bird toy) that we put on the door knob when he wants to go out. We would makes lot of noise w/ the bell before we would take him out & say "poopie" "poopie" and now he (usually...lol) rings the bell and sits by the door until we take him out.
I tried training Diamond that, but she never got it - she just runs to the door and looks back at you with a look of anticipation. The sleigh bells are still hanging on the front door, ten years later! LOL